First post, by tyuper
I would like to know if there's anyone who encountered the problem that manifests with full system lockup after entering command to start a game / an application when XMGR or HIMEMX are in use and MS-DOS is in HMA. At least that is happening on my two PC Chips A101's, one being in my possession for a long time and second I got recently in pristine condition.
With XMGR the only parameters that aren't causing aforementioned issue are /B (boot mode) and /N24 (number of XMS handles) and increasing /Nxx to anything above or just leaving without this parameter is a certain freeze. HIMEMX is out of question, no parameters that this manager has can dodge the issue. With one difference - while XMGR gives absolutely no hint, HIMEMX informs about "A20 hardware error". So I went with multimeter to explore motherboard's traces and discovered that A20 pin from HT6542B (keyboard controller) goes high after DOS boot-up and it goes low few moments later when ENTER is pressed to run a program. That doesn't happen with HIMEM, which BTW works perfectly. So I started to dug deeper and put this onto schematic:
I wonder if that SMIACT is the culprit, because on multimeter (cheap thing without True RMS) it shows ~3.5V, so it's surely enabling and disabling OR gate. Also couldn't find any schematics for Socket 7 motherboard, so no knowledge about how this A20 circuit is implemented on other motherboards. Even Intel datasheets don't tell much and starting with PIIX4 the logic is much simpler (there's A20GATE input from keyboard controller and A20M# output to CPU).
If anyone has PC Chips Socket 7 mobo with Intel chipset, then please let me know if those XMS managers work on your setup! Thanks!